THE APPALACHIANS: AN ACCRETIONARY AND COLLISIONAL OROGEN

The Appalachian orogen is the product of three major orogenies and a complete Wilson cycle that began with the breakup of supercontinent Rodinia and ended with formation of supercontinent Pangaea. The diachronous Taconian event (495–445 Ma) involved arc and microcontinent accretion, and corresponding deformation, metamorphism, and plutonism, along the Iapetan margin from Alabama to Newfoundland. Ordovician clastic wedges are correspondingly diachronous. The Sevier basin in the south and the foreland basin in Newfoundland formed early, whereas the central Appalachian Martinsburg clastic wedge may have formed about the same time, but achieved its greatest thickness during the Llandeilo and Caradoc. The Acadian-Neoacadian event (409–350 Ma) obliquely closed the Rheic remnant ocean as Avalon and Carolina superterranes subducted assembled Taconian and Laurentian elements, producing deformation, abundant I– and S–type plutons, high-grade metamorphism, and wholesale melting of the southern Appalachian Inner Piedmont. The latter formed a SW-directed orogenic channel between the primordial Brevard fault and central Piedmont suture. The corresponding SW-younging (Late Devonian-Mississippian) clastic wedge from Pennsylvania to Georgia tracks the progressive SW migration of uplift of the internides during transpressive, zippered collision. The Alleghanian orogeny (335-265 Ma) is correspondingly diachronous, producing early dextral strike-slip faulting throughout the orogen followed by 325 Ma metamorphism in the southern and central Appalachians, 335 and younger pluton emplacement, then Early Permian NW-directed thrusting in the southern and central Appalachians and metamorphism in SE New England reflecting the transpressional-rotational nature of the collision of Laurentia with Gondwana. The Alleghanian clastic wedge is correspondingly diachronous with early components in the N and S and later components forming in the central portions of the orogen. All of the clastic wedges contain detrital zircons from the previous—not the current—orogeny (Thomas' Law), except for the Permian Dunkard Group that contains an Alleghanian component. The Appalachian Wilson cycle is thus a composite of three orogens: one accretionary and two collisional.